The present invention relates to a process for the determination of the state of physical properties of rechargeable electric energy storage means, and an apparatus for carrying out the same.
Rechargeable (reversible) electric energy storage means are employed in very many areas today in order to ensure continued operation of electric devices without dependence on the mains supply. The presently most common types are lead, nickel-cadmium, sodium-sulphur, and lithium-fluor rechargeable batteries. Besides their many advantages. there are also drawbacks, for example, higher costs and greater maintenance requirements. The principle disadvantage, however is that the supply of energy is limited. Nonetheless, the fact that the tappable energy is limited and that the utilized rechargeable battery has to be replaced or recharged at some point is one that can be tolerated. The real problem is that one often does not know when that will be, i.e. how long one's device can still be operated with the rechargeable battery in use (how much energy/charge remains is in the battery).
In some fields of application these disadvantages are only a nuisance or expensive; in many cases they can, however, preclude the use of rechargeable batteries. In this case non-reversible, polluting primary cells are employed, which are more expensive and highly polluting after some use (number of cycles), but which present fewer problems regarding the charge data since the advanced condition of discharging becomes noticeable in time due to the drop in voltage. Fields in which safety is crucial, such as in medicine, traffic control, fire prevention, etc., and also many applications in hobbies (e.g. diving, mountain climbing, model-building) are slow in starting to employ rechargeable batteries or have hitherto not used them at all.
The reason for this is the behavior of most rechargeable batteries, as illustrated in FIG. 1 using the common Ni-Cad rechargeable batteries as an example:
Prior to the sudden, sharp drop in voltage just before total discharge there is a long phase in which the voltage is practically constant; during approx. 90% of the duration of the discharge (at constant current) the voltage only varies less than 10%. This property is initially a favorable state since practically constant voltage during nearly the entire duration of operation of the rechargeable battery ensures total efficiency of the device during this time. On the other hand, point C in the illustrated discharge curve, namely the point at which recharging is necessary does not become apparent until only substantially less than 10% of the overall charge is left following which voltage supply soon breaks down completely.
The only properties which can be measured without detriment to the rechargeable battery are magnitude of current and temperature, which are predetermined by the environment, and the voltage, which is at the electrodes of the rechargeable battery under these conditions. The nominal voltage of a rechargeable battery depends on its age and properties, which are subject to the statistical specimen deviation of a production series (hereinafter called statistical specimen deviation). As the change in voltage in the main part of the discharge process lies in the same magnitude as the mentioned dispersion of the nominal voltage, the voltage alone can only be a very poor measure of the charge/energy content of the rechargeable battery. Systems which derive the required data from the measurable properties are therefore necessary in order to determine the state of charge.
In the hitherto prior art methods of monitoring the charge condition, the easily measured properties of the process (current I, temperature T, voltage U and time t), which balance the charging/discharging right from the beginning of the process, are utilized with the aid of physical formulas (DE-PS 34 29 145) to estimate the state of charge.
None of the prior art methods has been adequate in practice. The reason lies in the unsatisfactory accuracy of measurement, and the marginal conditions that must be maintained for useful application:
The prior art processes/devices require a new battery to start the state of charge monitoring process; hitherto it has been impossible with previously used batteries. In particular with Ni-Cad cells it frequently occurs that batteries of unknown origin are employed. In this event monitoring in almost all cases is based on false input data, and therefore the determination of the available capacity must be incorrect.
During the lifetime of a battery, voltage must never reach zero, otherwise the monitoring unit will lose its memory and the following recharging monitoring will be based on a presumed new battery.
However, satisfactory accuracy of measurement is not ensured even if all the required marginal conditions are met, because, the indirect determination of the state of charge is based on easily measured values of dependencies in a table, but without taking into consideration either the individual manufacturing tolerances or the differences specific to type and manufacture.
The object of the present invention is therefore to design a process of determining the physical properties of rechargeable electric energy storage device, and apparatus for carrying out the same, with the input values of the process of the energy storage device being measured and processed in a computer in such a manner that even with energy storage means whose state of charge and origin is unknown a reliable determination can be made concerning its available capacity, age and efficiency.
The foregoing object is solved in accordance with the present invention which is predetermined model. Closed linear or non-linear multiparametric functions and/or a heuristic parametering are used to estimate the to-be-determined physical properties and their physical relationships to one another and the estimated values are compared with the measured values. The model and/or estimation process is then adapted for the succeeding measurement.
An introduction to the principle of measuring with observers is known from IEEE Transactions on Automatic Control, Vol. AC-16No. 6 Dec. 71, pages 596-602, "Introduction to Observers" by David G. Luenberger or from IEEE Transactions on Military Electronics, Vol. 7, 1963, David Luenberger: "Observing the State of a Linear System", pages 74-80, Prof. Zeitz, Michael, Bochum, "Nichtlineare Beobachter . . . " (Non-linear Observers), VDI-Verlag, Fortschrittberichte VDIZ, Reihe 8, NR. 27.
Indirect measuring employing models results in values that cannot be measured directly. A comparison of these values with the real values of the process is at the most possible at some later point (e.g. upon reaching complete discharge). Prior to this, however, undetermined influences can alter the process so that an initially unrecognizable measurement error occurs in the indirectly measured value.
For instance if the voltage is measured at the electrodes in the central region of operation of the rechargeable battery, it is hardly possible--even if the data of a closed model is known--to derive data about the state of charge of the rechargeable battery if it is a type of rechargeable battery which has an even voltage characteristic curve.
Only with knowledge of two essential properties is voltage a significant characteristic for the state of charge. These are the individual statistical specimen deviation and the state of aging. The variations in the voltage resulting therefrom lie in the same magnitude as those resulting from the change in the state of charge.
A direct measurement, e.g. according to DE-OS 3736481 is based on a new rechargeable battery, or a rechargeable battery of known age as well as on catalogue specifications for the individual energy storage. Individual deviations from the catalogue specifications are not taken into consideration just as deviations of the assumed age from the actual age.
The result is information about the state of charge of the rechargeable battery, which in an extreme case deviates very greatly from reality, without the measuring system receiving feedback about this deviation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.